1. Field of the Invention
This invention relates to a pigment composition, more particularly to a surface-treated pigment composition, a surface-treated pigment dispersion and a method of treating pigment particles.
2. Description of the Related Art
It is known in the art to surface treat pigment particles in order to improve the dispersing properties of the pigment particles in a liquid such as a binder resin or polymer solution. There are methods which utilize high molecular weight polymers to treat the surface of pigment particles by forming a coating on the surface of pigment particles. An example of the high molecular polymer dispersing agent is disclosed in JP 5320533. Generally, the high molecular weight polymers are attached to the surface of pigment particles via adsorption.
U.S. Pat. No. 6,548,174 B2 discloses a pigment dispersing resin having a weight average molecular weight within a range of 5000-100,000 which is obtained by copolymerizing a macromonomer with an ethylenically unsaturated monomer. Preparation of a macromonomer is necessary to produce such a high molecular polymer dispersing resin. This patent discloses an epoxy-containing polymerizable unsaturated monomer, such as glycidyl methacrylate, for use in preparing the macromonomer.
U.S. Pat. No. 5,969,048 discloses a colored coating composition which comprises (a) a polymer having a carboxyl group and a carboxylate group, (b) a polymer having a hydroxyl group and an epoxy group, (c) a pigment-dispersing agent, and (d) a pigment. The polymer (b) disclosed therein is a high molecular weight copolymer which is prepared by copolymerizing an epoxy group-containing ethylenically unsaturated monomer with a hydroxyl group-containing ethylenically unsaturated monomer and the other copolymerizable ethylenically unsaturated monomer.
Surface-treatment agents containing silicon, such as silylating, or silane coupling agents are generally attached to the surface of pigment particles through a chemical bond. For example, Japanese Publication No. JP 7-331102 discloses a method of treating an organic pigment with a silylating or silane coupling agent. An example of the silylating agents disclosed therein is glycidoxypropyl-trimethoxysilane which includes an epoxy group.
U.S. Pat. No. 6,120,596 discloses a method of treating pigment particles by contacting a dispersion of pigment particles with halosilane or organohalosilane, elevating the temperature to form a siloxane on the surface of the particles, and recovering the surface treated particles as a presscake or as dry particles.
U.S. Pat. No. 5,719,206 discloses an aqueous coating composition which includes an inorganic particulate substance whose surface is modified with a silane coupling agent having at least one functional group selected from a group consisting of a vinyl group, a methacryloyl group, epoxy group, and an amino group; and a water-dispersible resin having a functional group which is reactive with a functional group of the silane coupling agent. In an example of said patent, glycidoxypropyl-trimethoxysilane is used as the silane coupling agent having an epoxy group.
While the silicon-containing surface-treating agents such as alkoxysilanes provide the treated pigments with improved physical properties, such as high heat and chemical resistance, good dispersing ability, good weather resistance, etc., however physical properties of the film resulted from the dispersions of such treated pigment particles are unsatisfactory because of insufficient compatibility of the treated particles to a binder resin due to the presence of silicon.
Conventionally, a color filter for a flat display includes a coloring layer formed from a composition containing a pigment dispersed in a binder. In order to provide a color filter with good light transmittance characteristics, it is desirable to provide a pigment composition which has an excellent pigment dispersing property, small pigment particle size, high dispersion stability and good compatibility among the components contained in the composition.